Integrated circuits are produced on a substrate, such as a silicon wafer, by patterning regions in the substrate and by patterning layers on the substrate. Deposited layers of conductive material are an integral part of every integrated circuit, and commonly provide the role of wiring. Such conductors, also referred to as "lines" or "runners," are typically provided at different elevations within the wafer structure. At some locations it becomes necessary to make electrical contact between higher and lower elevation conductive lines on the wafer. This invention concerns a problem associated with making low resistance electrical contact between two different elevation conductive layers. The hitherto unrecognized problem arises when the device contains titanium at the junction of the two levels of metalization and the junction is annealed in a reducing atmosphere. The solution to the problem, however, has broader applicability than just to interlevel metal junctions.
Aluminum is a common material employed as the metalization for lines due in large part to the ease with which it can be deposited and worked and due to its high conductivity. Most commonly, the aluminum materials utilized are alloys of aluminum and copper comprising greater than 95% aluminum. Aluminum lines are often coated with a thin film of titanium or titanium nitride. Although they are highly conductive and readily deposited, lines made from aluminum and its alloys have the one drawback that they readily form aluminum oxide on the surface when exposed even briefly to an oxidizing atmosphere such as air. Aluminum oxide is a highly insulative material which, if left unreduced, leads to a high resistance junction. With increasingly refined design rules for semiconductor devices, the diameter of the connecting hole created in the insulating layer for establishing connection between an upper and lower metalization must be minimized. As the diameter of the contact decreases, the importance of low resistance increases.
The deposition of a layer of titanium metal on top of the aluminum metalization is known in the art (see U.S. Pat. No. 5,200,359). When exposed to an appropriate energy source, the titanium metal is made to react with aluminum oxide on the surface of the aluminum, the aluminum oxide is reduced to aluminum and the titanium scavenges the oxygen. In this process some titanium dioxide appears to be produced at the interface, but it has a much lower electrical resistance than does aluminum oxide and therefore produces a junction of lower resistivity. In addition, a certain portion of the oxygen is transported away from the junction and into the titanium metal layer.
A constraint in fabricating integrated circuits is that so-called "defect states" in dielectrics must be passivated in order to minimize unacceptable electrical leakage. The solution to this problem has been found to involve annealing the device in a reducing atmosphere whereby hydrogen appears to attach to the dangling bonds, thereby greatly reducing stray conductivity.
It has now been discovered that the annealing in a hydrogen-containing atmosphere that is desirable for reducing or passivating the defects states interferes with the titanium's ability to act as a reducing agent. Consequently, the conductivity of metal-metal junctions is reduced.